Any ideas as to what this is?

[Viperjim1]

I tested this loco as I’m not into HO scale and it’s a brass pacific fast Mail gs-4 and under dc power it just made like a crackling noise. And came from the tender. After disassembly of the tender I found this contraption and as you can see it has a sound vent or holes in the bottom of the tender. The loco did not move , just crackled and stopped.
Hooked it up to my dcc to see if it was dcc as I don’t want to take the loco apart but nothing. Not even a crackle! So wondering what it is. [ Guests cannot view attachments ] i

[Rasputen]

That looks like a homemade DC steam sound device, I think I have the plans for one somewhere.

[Viperjim1]

Thanks for the info.

[tpwillie]

The silver case is a speaker for the original sound unit that PFM brought out in 70's that was controlled by a proprietary system that was designed by PFM.  The original PFM system was large metal cased transformer, sound generator, and other special functions.  Was expensive and for a first attempt to get sound on MRR equipment it was a valiant effort, but costs and lack of smaller components it never really reached its' potential or a large market.  It was mainly steam for the sounds and was kind of fun when there wasn't really anything else available.  The speaker was top quality and the ported enclosure did a great job of producing sounds with technology that was way behind what we have in todays' DCC world.  There is probably a cam on one driver that created the chuffs for the cylinder exhaust.  The power for the sound went from the throttle on the case thru the rails.  DCC in its' infancy. 

John (tpwillie)

[peteski]

Interesting . . .
I wonder if the sound (chuffs) were generated in that PFM "box" and the chuff frequency was proportional to the track voltage (not directly synchronized with the drivers). The audio was then send to the loco through the rails (by AM modulation of high frequency voltage injected into the track voltage, and the electronics in the tender just filtered the sound portion of the voltage at the rails and played it through the speaker.  Similar to the constant lighting circuits popular around that time, except this was "audio through the rails".  This is just pure speculation on my part.

If you know for sure that there was a cam in the loco, then I suspect that the PFM box was just like those constant lighting circuits, and it supplied high frequency voltage through the track to the sound generator in the model which they rectified it, and powered the on-board sound generator.  That probably consisted of a white noise generator, audio amp, and some sort of audio-attenuation coupled to the chuff cam to produce those "chiuff" bursts of white noise synchronized with the drivers.

[Mike C]

Interesting . . .
I wonder if the sound (chuffs) were generated in that PFM "box" and the chuff frequency was proportional to the track voltage (not directly synchronized with the drivers). The audio was then send to the loco through the rails (by AM modulation of high frequency voltage injected into the track voltage, and the electronics in the tender just filtered the sound portion of the voltage at the rails and played it through the speaker.  Similar to the constant lighting circuits popular around that time, except this was "audio through the rails".  This is just pure speculation on my part.
   I drooled all over th PFM system as a teen and they were all cam operated .,..Mike
Not a valid %s URLIf you know for sure that there was a cam in the loco, then I suspect that the PFM box was just like those constant lighting circuits, and it supplied high frequency voltage through the track to the sound generator in the model which they rectified it, and powered the on-board sound generator.  That probably consisted of a white noise generator, audio amp, and some sort of audio-attenuation coupled to the chuff cam to produce those "chiuff" bursts of white noise synchronized with the drivers.

[Mike C]

I hate texting !  I drooled over the PFM system in the 70's and definitely remember them being cam operated ...Mike

[peteski]

I hate texting !  I drooled over the PFM system in the 70's and definitely remember them being cam operated ...Mike

Thanks for the info Mike. So the "box" likely provided power to the circuit inside the loco.  Probably similar to the high frequency signal for constant lighting.  It is interesting to discover what kind of technology was being utilized before DCC.

[learmoia]

Thanks for the info Mike. So the "box" likely provided power to the circuit inside the loco.  Probably similar to the high frequency signal for constant lighting.  It is interesting to discover what kind of technology was being utilized before DCC.

The owner of our LHS runs a DC Narrow Gauge Railroad with PFM Sound....

From what I understand... the speaker is inside the locomotive.. but the 'sound system' is stationary like a second throttle that lets you control and fine tune the tone and volume of multiple noises plus the bell and whistle.
Chuff is controled by a cam..

Some how there is a multi-track cassette tape involved.. and you can change tapes for different locomotives.

And I may be wrong, but I recall the sound works even if the locomotive is stopped... So the Sound system sends it's own signals through the rail, and there maybe a filter on the motor to filter out the sound commands...

The best I can figure, the 'sound' is sent from the base station to the speaker on the locomotive through the rails on top of the DC Voltage... maybe very similar to how DCC signals are sent through with the voltage to talk to decoders.

... Well I was somewhat correct..
https://modelrailroadforums.com/forum/index.php?threads/pfm-sound-system-how-did-that-work.10783/

~Ian

[peteski]

Thanks for finding that thread Ian.  Sounds like I was on the right track.

It simply put another signal onto the tracks. One had to use RF chokes to keep the high frequency out of DC things (motor), and capacitors to keep the DC out of the speakers. A cool side effect was that one could mess around with the RF chokes and capacitors to get constant intensity lighting from it too. Hence the reason PFM was one of the first to start using low current 1.5v lamps for headlights.